L-Tyrosine enhancement of the elevation of 3,4-dihydroxyphenylacetic acid concentration in rat brain by spiperone and amfonelic acid

Elevation of rat brain tyrosine levels by phenelzine is mediated by its active metabolite β-phenylethylidenehydrazine

Phenelzine, a non-selective irreversible inhibitor of monoamine oxidase (MAO), has been used in the treatment of depression and anxiety disorders for several decades. It is a unique inhibitor of MAO as it is also a substrate for MAO, with one of the metabolites being β-phenylethylidenehydrazine (PEH), and it also inhibits several transaminases (e.g. GABA transaminase) in the brain when administered i.p. to rats. Administration of either phenelzine or PEH to rats has been reported to produce dramatic increases in rat brain levels of GABA and alanine while reducing levels of glutamine; these effects are abolished for phenelzine, but not for PEH, when the animals are pre-treated with another MAO inhibitor, suggesting that they are mediated by the MAO-catalyzed formation of PEH from phenelzine. In the present report, we have found that phenelzine and E- and Z-geometric isomers of PEH significantly increased rat whole brain concentrations of L-tyrosine. In a time-response study, acute administration of phenelzine, E-PEH and Z-PEH (30 mg/kg i.p.) elevated rat whole brain L-tyrosine levels at 3 and 6h following injection, reaching approximately 265-305% of vehicle-treated controls at 3h. To determine whether the effect on L-tyrosine is MAO-dependent, animals were pre-treated with the non-selective MAO inhibitor tranylcypromine (1mg/kg i.p.) prior to administration of phenelzine, racemic PEH or vehicle controls. This pre-treatment reversed the effects of phenelzine, but not of PEH, on brain L-tyrosine levels, suggesting that the tyrosine-elevating property of phenelzine is largely the result of its active metabolite PEH. These results are discussed in relation to possible therapeutic applications of these drugs.

Clozapine-induced dopamine release in the medial prefrontal cortex is augmented by a moderate concentration of locally administered tyrosine but attenuated by high tyrosine concentrations or by tyrosine depletion

RATIONALE

Tyrosine availability can affect indices of dopamine (DA) release in activated central DA systems. There are, however, inconsistencies between studies. One possibility is that the relationship between tyrosine availability and DA release is non-linear.

OBJECTIVES

This study aimed to determine how tyrosine depletion as well as a range of administered tyrosine concentrations affect antipsychotic drug-induced extracellular DA levels in the MPFC or striatum.

METHODS

A guide cannula was implanted over the medial prefrontal cortex or striatum of adult male rats. After a 24-h recovery period, a microdialysis probe was inserted. Microdialysate collection began on the following day. Some rats received vehicle or a tyrosine- and phenylalanine-free neutral amino acid solution NAA(-) (IP) prior to clozapine (CLZ 10 mg/kg IP). Others received vehicle, CLZ (10 mg/kg IP) or haloperidol (HAL) (1 mg/kg IP) while the probe was perfused with artificial cerebrospinal fluid containing tyrosine 0-200 mug/ml.

RESULTS

NAA(-) reduced tyrosine levels in MPFC dialysate by 35%. This reduction did not affect basal MPFC DA levels but attenuated the peak of CLZ-induced MPFC DA levels. The NAA(-) effect could be reversed by administration of tyrosine. Infused tyrosine 12.5-200 mug/ml did not affect basal DA levels either in MPFC or striatum. Within the MPFC, tyrosine 50.0 mug/ml significantly increased CLZ-induced DA levels. Within the striatum, tyrosine 25.0 mug/ml significantly increased while 150.0 mug/ml significantly decreased HAL-induced DA levels.

CONCLUSIONS

Basal extracellular levels of DA in the MPFC and striatum are not affected by wide changes in tyrosine availability. However, modestly increased brain tyrosine levels can augment CLZ-induced MPFC and HAL-induced DA levels. Very high tyrosine concentrations attenuate HAL-induced striatal DA levels. These data may explain inconsistencies in the literature and suggest that tyrosine availability could be exploited to modulate psychotropic drug-induced DA levels in the brain.

Dopamine release in rat striatum: physiological coupling to tyrosine supply

Intracerebral microdialysis was used to monitor dopamine release in rat striatal extracellular fluid following the intraperitoneal administration of dopamine's precursor amino acid, L-tyrosine. Dopamine concentrations in dialysates increased transiently after tyrosine (50-100 mg/kg) administration. Pretreatment with haloperidol or the partial lesioning of nigrostriatal neurons enhanced the effect of tyrosine on dopamine release, and haloperidol also prolonged this effect. These data suggest that nigrostriatal dopaminergic neurons are responsive to changes in precursor availability under basal conditions, but that receptor-mediated feedback mechanisms limit the magnitude and duration of this effect.

Acute effects of aspartame on large neutral amino acids and monoamines in rat brain

The dipeptide aspartame (APM; aspartylphenylalanine methylester), an artificial sweetener, was studied in vivo for its ability to influence brain levels of the large neutral amino acids and the rates of hydroxylation of the aromatic amino acids. The administration by gavage of APM (200 mg/kg) caused large increments in blood and brain levels of phenylalanine and tyrosine by 60 minutes. Brain tryptophan level was occasionally reduced significantly, but the brain levels of the branched-chain amino acids were always unaffected. Smaller doses (50, 100 mg/kg) also raised blood and brain tyrosine and phenylalanine, but did not reduce brain tryptophan levels. At the highest dose (200 mg/kg), APM gavage caused an insignificant increase in dopa accumulation (after NSD-1015), and a modest reduction in 5-hydroxytryptophan accumulation. No changes in the brain levels of serotonin, 5-hydroxyindoleacetic acid, dopamine, dihydroxyphenylacetic acid, homovanillic acid, or norepinephrine were produced by APM administration (200 mg/kg). These results thus indicate that APM, even when administered in amounts that cause large increments in brain tyrosine and phenylalanine, produce minimal effects on the rates of formation of monoamine transmitters.